1. Field of the Invention
This invention relates to integrated circuit structures. More particularly, this invention relates to an improvement in the formation of a planarization layer for an integrated circuit structure.
2. Description of the Prior Art
In the construction of integrated circuit structures, the use of a photoresist to form a mask over nonplanar underlying layers having steps can result in line width deviations due to thin film interference effects or as a result of bulk light absorption differences due to variations in photoresist thicknesses. To achieve a substantially planar photoresist surface, it has been proposed to coat the underlying topography of the integrated circuit structure with one or more polymer layers to achieve at least partial planarization before application of the photoresist layer.
For example, L. K. White, in "Planarization Properties of Resist and Polyimide Coatings", Journal of the Electrochemical Society Solid State Science and Technology, July, 1983, at pages 1543-1548, discusses multilayer planarization wherein a polymer dissolved in a solvent is applied to a wafer by spin coating. Two layers may be spun on at different speeds, e.g., 4000 and 7000 rpms, to attempt to achieve a more planar surface to which the photoresist may be applied. However, the author states that only slight improvements in the spun on planarization properties results through the use of such a multiple coating technique. Apparently this is due, at least in part, to the tendency of the second coating to mix or dissolve a portion of the initial coating to thereby increase the viscosity and per cent solids composition of the second polymer solution at the interface between the coatings. The author further discusses planarization techniques involving a positive photoresist polymer exposed to white light and then baked to obtain thermal flow.
Planarization using solvent based polymers presents additional difficulties in achieving the desired planarization due to the tendency of the solvent to evaporate during the application of the polymeric material to the integrated circuit wafer by conventional spinning techniques. This evaporation, in turn, changes the solution viscosity and, thus, changes the flow rate as the polymer spreads out on the spinning wafer. Furthermore, the subsequent evaporation of the solvent (after it has been spun across the entire surface of the wafer) involves a reduction of the total volume of the planarization material which, in turn, results in nonuniformity of the coating since thicker portions of the coating (over low points in the topography of the underlying integrated circuit structure) contain more solvent and, therefore, more solvent will be lost resulting in a larger reduction of the volume of the planarization coating at that point. Conversely, a lesser amount of coating over a high point on the integrated circuit structure results in much less solvent evaporation and, therefore, a smaller reduction in the volume content of the remaining polymer.
It would, therefore, be most desirable to use a planarization material which could be spread smoothly over an integrated circuit structure having varying topography wherein the material could be easily spread in liquid form across the surface without being subject to the variation in thickness due to solvent evaporation as in the present practices.